Method and apparatus for drying sheet materials

ABSTRACT

Wood veneers or other flat materials are passed through successive sections of energized meander waveguides and through jets of heated air from air knives situated between the waveguide sections to effect a very rapid drying action in which relatively moist areas are dried more strongly than dryer areas to produce a finished product of uniform moisture content. The waveguide and air knife structure is disposed within an electrical conductor walled, thermally insulated cabinet with continuous feed conveyor means and an air recirculation system in an arrangement which optimizes operating efficiency and convenience.

United States Patent [72] inventors Franklin J. Smith Diablo; KlausSilbermann, Danville, both of Calif. [21] Appl. No. 6,499 [22] FiledJan. 28, 1970 [45] Patented Nov. 23, 1971 [73] Assignee CryodryCorporation San Ramon, Calif. Original application Nov. 14, 1967, Ser.No. 682,903. Divided and this application Jan. 28, 1970, Ser. No. 6,499

[54] METHOD AND APPARATUS FOR DRYING SHEET MATERIALS 7 Claims, 12Drawing Figs.

[52] US. Cl 219/1055, 219/ 10.69 [51] Int. Cl 1105b 9/06, 1105b 5/00[50] Field of Search 219/1055 [56] References Cited UNITED STATESPATENTS 3,235,702 2/1966 Timmermann et a1 219/1055 23 23 23 EATED AIRHEA ED A R 3,304,399 2/1967 Timmermann et a1 219/1055 3,413,483 11/1968Timmermann et a1 219/1055 3,449,836 6/1969 Schreiber et al 219/10553,471,672 10/1969 White 219/1055 3,474,210 12/1969 Silbermann et a1.219/1055 3,474,213 10/1969 Hilton et a1. 219/1055 3,507,050 4/1970 Smithet a1 219/1055 X Primary Examiner-J. V. Truhe Assistant Examiner-1. H.Bender Attorneys-Carl C. Batz and Fryer, Tjensvold, Feix, Phillips andLempio MICROWAVE MICROWAVE TMC ROWAVE HEATED AIR PATENTEDHUY 2 I 71 3.622 7 3 3 saw 1 or 7 MICROWAVE MICROWAVE HEATED AIR) HEATED AIR HEATEDAIR M lCROWAVE III INVHNTORS FRANKLIN J. SMlTH KLAUS SILBERMANN ATTORNEPATENTEDNnv 23 ISYI SHEET 2 BF 7 FIG 3 INVI'INTORS FRANKLIN J. SMITHKLAUS SILBERMANN WM/Z' ATTORNE PATENTEDunv 23 I971 SHEET 3 BF 7 me vm mmv wz - mvramons FRANKLIN J. 3mm

KLAUS SILBERMANN M 9 AT TORN PATENTEDSnv 23 19h SHEEI b [1F 7 FIG 5INVFN'H )RS FRANKLIN J, SMITH KLAUS SILBERMANN Wag ATTORNEY PATENTEDNuv23 ml 3.622.733

SHEET 5 BF 7 INVI'INTORS F G 6 FRANKLIN .1. SMITH KLAUS SILBERMANNATTORNEY PATENTEDunv 2 3 an SHEET 8 OF 7 INVENTORS FRANKLIN J. SMITHKLAUS SILBERMANN ATTORNEY PATENTEU 2 3, 622 7 33 FIG 1O INVENTURSFRANKLIN J. SMITH KLAUS SILBERMANN ATTORNEY METHOD AND APPARATUS FORDRYING SHEET MATERIALS This application is a division of the applicationby Smith et al., Ser. No. 682,903 entitled Method and Apparatus forDrying Sheet Materials, filed Nov. 14, 1967.

a This invention relates to the drying of sheet materials and moreparticularly to a method and apparatus for rapidly and efficientlydrying such materials with microwave energy on a continuous processbasis.

. Many industrial products which must be dried in the course ofmanufacture have a thin, flat configuration and may be treated byspecialized techniques which differ from those employed for processingbulky objects, ,granular materials or products having otherconfigurations. Wood veneers of the type which are laminated together toform plywood are an example of a sheet material which is customarilysubjected to a drying operation. The present invention was initiallydeveloped to facilitate the drying of veneers and accordingly will bedescribed with reference thereto, it being understood that the'inventionis also applicable to the drying of many other similarly shapedproducts.

Veneers are typically about 1/10 inch to 7/32 inch thick and are about50 inches wide and 8 feet long. To form plywood, several of the driedveneers must be bonded together and it is preferable'that a hot resin beused for bonding and that pressure be applied to the veneers as theresin hardens. Product quality is very dependent upon the moisturecontent of the veneers at the time of bonding. The moisture contentshould typically be from about 3 percent to about 8 percent and it isimportant that this moisture content be as uniform as possiblethroughout all areas of each veneer.

If there are localized areas of relatively high moisture content in anyof the veneers during bonding a phenomenon known as blow-up may occurwhen the pressure is relieved. This is a physical disruption of theadjacent surfaces of the veneers at the wet areas and is believed toresult from steam pressure generated at themoisture concentration.Conversely, if there are localized areas which have a moistureicontentbelow the optimum range, checking, brittleness and loss of strength mayresult. The problem of moisture leveling, i.e., obtaining'a uniformmoisture content throughout the veneers, has been particularlytroublesome in the plywood industry.

' While veneers have been dried by being pressed between heated platensand by various other means, the technique most widely employed atpresent makes use of an air drying chamber. The usual structure includesa cabinet in which roller conveyors, known as nip rolls, carry theveneers through the drying, region. Heated air is continuously passedthrough the cabinet and insome machines, air knives are used to directhigh velocity jets of air against the veneer surfaces. These aretubesinto which heated air is pumped and which are disposed transverse to thepath of the veneers and in close proximity'thereto. The tubes areusually arranged in pairs so that the veneers pass between the twomembers of each pair. Each tube has a long slit opening facing theveneers to direct the jet of heated air thereagain'st.

Several problems are encountered in utilizing an air dryer of this kindfor drying veneers. Processing time, for example, is undesirably long.Drying, in these systems, is dependent on heat transfer from the surfaceof the veneers into the interior thereof and several factors limit therate at which such heat transfer can be made to take place. Inparticular, the surface temperature of the veneers'm ust be kept belowafixed limit to avoidscorching or otherwise degrading the wood. Secondly,wood is a poor thermal conductor, particularly after partial drying. Inconventional systems, these factors have resulted in minimum processingtimes of the order of minutes. If the air temperature is maintainedsufficiently low to avoid any adverse effects on product quality, muchlonger processing time is required.

Another consequence of long processing times is that several parallelcolumns of veneers must be passed through the dryer at any given time,if reasonably efficient operation is to be realized without an extremelylong dryer. This in turn requires multiple conveyor systems and complexproduct input and output mechanisms.

Still another major problem area in the use of conventional air dryersis that it is very difficult to obtain dried veneers with the preferreduniform moisture content. Green veneers may differ in their overallmoisture content and localized areas of each individual veneer may havedifferent moisture contents. Knots, for example, are typically moremoist than the surrounding areas of the veneer. Using conventionalsystems, the relatively moist veneers and the'relatively moist areas ofindividual veneers, remain in' this condition after drying. Veneerswhich have an uneven moisture distribution, or an overall moisture leveloutside of the desired range, are detected at the output of the dryerand must be recycled or rejected. Successful recycling is, in turn, verydifficult. In reducing the moist areas to the desired range, the otherareas of the veneers tend to become too dry. This problem of moistureleveling has been a major source of operating inefficiency and processcomplication, not only in the drying of wood veneers but in drying manyother sheetlike products as well.

To avoid certain of the difficulties discussed above, microwave heating,rather than air drying, may be used for treating various materials. Thisform of heating is unique in that it does not depend upon heat transferinward from the surface of the material but very rapidly generates heatdirectly .within the interior region thereof. Further, microwave energyinteracts primarily with the internal water content of the material."This provides for a moisture leveling-effect in that proportionatelymore heat is generated in the regions of the material which contain themost moisture. However, the treatment of industrial products withmicrowave is a relatively new art and the techniques and equipment whichhave heretofore been utilized are not optimally adapted to the drying ofveneers or many other sheetlike materials.

A sheet material can be heated rapidly by being passed through anenergized waveguide. If the waveguide is energized in one of certainmodes, longitudinal slots can be provided along'the' sides of thewaveguide to provide for passage of the sheet material therethrough.Further the material may be passed through successive sections of asingle waveguide while traveling along a linear path if the waveguide isformed with a serpentine or meander configuration which repeatedlytransects the ,path. In practice, there are many serious problemsinvolved in efficiently utilizing such a system for the drying ofveneers and the like.

Microwave irradiation both heats the veneers and tends to drive themoisture thereof outward toward the surfaces of the material. Thisresults in the formation of a layer of moisture at the surfaces whichcan adversely affect the drying operation in several respects. First,the surface moisture concentration must be dissipated if drying is to beaccomplished and if evaporation by microwave heating is relied upon forthis purpose, a very high microwave power input or undesirably longprocessing time is required. The process is further complicated in thatsuch evaporation tends to maintain the surface regions of the veneers ina relatively cool condition.

A very significant problem which is encountered in meander waveguidedrying systems arises from the fact that the microwave. energy densityis at a maximum at the input end of the waveguide and diminishes in anonlinear manner therealong. Since in the meander configuration thesuccessive sections of the waveguide extend transversely relative to theveneers, each waveguide section is characterized by a diminishing energydensity in a direction transverse to the veneers, with the heatingeffect being greatest at the end of each waveguide section whichis'electrically closestto the power input. This energy density gradientis reversedat each successive transverse section of the meanderwaveguide and thus each suchsection provides some compensation for theuneven heating effect of the preceding sectionrl-lowever, this isinsufficient to fully compensate for the effect since the aggregateenergy density within each waveguide section is substantially less thanthat within the preceding section. The net effect is that an uneven heatinput may have occurred across the veneers after passage through theentire system with the maximum heating having occurred. along the edgeof the veneers which was adjacent to the power input end of the meanderwaveguide.

Still other difficulties are encountered in making efficient use of themeander waveguide system in connection with conveying and'supporting thethin semiflexible veneers within the necessarily intricate apparatus andwith respect to matching the localized energy density along thewaveguide with the progressive decrease in the moisture content of thewood as it passes through the system. Further, difficulties areencountered with respect to suppressing the release of microwave energyfrom the system, with electrical arcing and in providing for convenientaccess to various regions of the apparatus.

This invention provides apparatus and techniques which resolve theproblems discussed above by providing for drying veneers and other sheetmaterials in a fast, convenient, and highly efficient manner and underconditions which produce a product of uniform moisture content. Theinvention uses a combination of microwave heating and air drying inwhich the veneers or the like are continuously passed through asuccession of spaced high intensity microwave regions which may besuccessive sections of one or more meander waveguides. Surface moistureis efficiently removed by a series of gas flows which are impingedagainst the veneers at spaced zones along the path of travel thereof.

In addition to effecting a very rapid and uniform drying, costefficiencies result, in that this system does not depend upon microwaveheating to supply all of the heat needed to vaporize the moisturecontent of the product. The microwave heating functions in part to driveinternal moisture toward the surface of the product where it may be moreeconomically removed by the gas flow.

The invention provides several structural innovations which jointlycontribute to the improved drying operation.

In a preferred form, the invention utilizes two separately energizedmeander waveguides which interleave in a structurally simpleconfiguration that matches the diminishing energy intensity along eachwaveguide with the progressive decrease in product moisture contentduring the course of drying. Further, and very importantly, theinterleaved multiple meander waveguide arrangement provides for powerinput to the two waveguides at opposite sides of the veneer path withthe result that the transverse energy density gradients discussed aboveeffectively compensate each other and a fairly uniform energy densityacross the veneers is realized for the system as a whole.

The gas flows between the successive waveguide sections are preferablyprovided by pairs of air knives situated therebetween and which may bephysically joined to an adjacent waveguide section to produce severalstructural and functional advantages. For example, a portion of the gasflow is caused to enter the adjacent waveguide section to scavengemoisture and debris therefrom.

According to still another aspect of the invention, the waveguide andair knife assembly is divisible approximately along the plane defined bythe path traveled by the veneers so that one portion of the assembly maybe separated from the other, preferably by motorized lifting means, toprovide access to the interior of the assembly for cleaning andmaintenance. To provide for such separation without manual disengagementof connectors, choke couplers of unique design are used to connect themovable portions of the waveguide assembly with the microwave source andwaveguide terminators.

In still another aspect of the invention the waveguide and air knifeassembly, together with a roller conveyor system, is disposed within acabinet having an air recirculation system associated therewith toreduce air heating costs. Preferably the cabinet is thermally insulatedto reduce heat losses and has electrically conducting wall members toconfine microwave energy which may be released from the waveguides.Veneer input and output passages at the ends of the cabinet haveelectrically lossy material therein to suppress energy emissiontherethrough.

A further aspect of the invention provides for lip projections along thelongitudinal slots in the waveguide wall through which the veneers passto provide for wide slots while inhibiting the release of microwaveenergy therethrough. Arcing at such slots is suppressed by specializedlip configurations.

The invention embodies several specific refinements of the featuresdescribed generally above as well as still other features to behereinafter described.

Accordingly, it is an object of this invention to provide a method andapparatus for more rapidly and efficiently drying wood veneers and othersheet materials.

The invention, together with further objects and advantages thereof,will best be understood by reference to the following specification inconjunction with the accompanying drawings of which:

FIG. I is a diagrammatic perspective view illustrating the drying of asheet material in accordance with the invention and showing certainelements of apparatus which may be utilized for this purpose;

FIG. 2 is a perspective view of a waveguide construction which may beutilized in the practice of the invention and which is characterized bya broad slot opening for passing the sheet material through thewaveguide while suppressing the emission of microwave energy therefrom;

FIG. 3 is a side elevation view of a veneer dryer embodying theinvention;

FIG. 4 is a plan section view of the veneer dryer of FIG. 3 taken alongline 4-4 thereof;

FIG. 5 is an elevation section view of the product input end of theveneer dryer taken along line 5-5 of FIG. 4;

FIG. 6 is an elevation cross section view of the veneer dryer takenalong line 6-6 of FIG. 4;

FIG. 7 is a section view taken along 77 of FIG. 5 illustrating adisengageable air coupling utilized in the veneer dryer;

FIG. 8 is a plan section view taken along line 8-8 of FIG. 6 showing achain drive system utilized for retracting certain elements of theveneer dryer from other elements thereof to provide access to interiorregions of the structure;

FIG. 9 is an elevation section view taken along line 99 of FIG. 6, withthe central region of the apparatus omitted, illustrating additionalinternal components of the veneer dryer including a conveyor system forcarrying veneers therethrough;

FIG. 10 is a perspective view of a choke coupling utilized forelectrically connecting relatively movable elements of the waveguidestructure without requiring a mechanical attachment therebetween;

FIG. 11 is a side elevation view of the choke coupling of FIG. 10; and

FIG. 12 is a plan section view of the choke coupling taken along linel2l2 thereof.

Referring first to FIG. 1, veneers 21 are dried, in accordance with thepresent invention, while being conveyed along a predetermined path asindicated by arrow 22 and by applying microwave energy to the veneers ata plurality of zones 23 which are spaced apart along the path of travel.In combination with these operations, high velocity flows of air orother gas are directed against the surfaces of the veneers at regions 24which are also spaced along the path of veneer travel. A very rapiddrying results in that the microwave energy penetrates into the veneersand interacts primarily with the internal moisture therein to generateheat throughout the in terior of the wood. In addition to the heatingeffect, this tends to drive the internal moisture towards the veneersurfaces.

One result of the microwave heating at zones 23 is that a layer of suchmoisture tends to form on the surface of the veneers. In the absence ofthe air flows at regions 24 this effect would slow the drying process asthe surface regions of the veneers tend to remain relatively cool,probably as a result of the cooling effect of evaporation at themoisture film. This slowing of the drying process is avoided in thepresent invention by the high velocity gas flows which are impingedagainst the veneers at regions 24 to break up and remove the superfcialmoisture layers. Thus, moisture which has accumulated at the veneersurfaces after each passage through a microwave zone 23 is removed fromthe vicinity of the veneer prior to the passage of the veneer through asubsequent microwave zone.

It is possible to use cold dry gas, generally air, at regions 24 and insome cases this is a preferred technique since heat induceddeterioration of the veneer surfaces is avoided and an extremely highquality product is obtained. However, it is usually preferable to employheated air to expedite the drying action and toreduce processing cost.By this means, a substantial portion of the heat needed for drying issupplied through the airflow.

This provides cost savings in that it is generally more economical toapply heat through a gas flow, to the extent possible, rather thanrelying on the more expensive microwave heating alone-However thetemperature of the air which is employed in the present invention may besubstantially lower than is the case in conventional heatedair dryers.Where the air temperatures in conventional dryers are typically in therange, from about 300 F. to about 450 F., the air in the presentinvention may typically be at about 225 F. Using the relatively low airtemperature in the present invention, deleterious effects on the veneersare minimal. Thus it should be understood that various airflowtemperatures may be used in the practice of the present invention asdetermined by the desired product qualityon the one hand andprocessing-time and cost considerations on the other hand. However, insubstantially all instances the air temperatures employed are lower thanthose which have been customary in conventional air dryers and productquality is thereby benefited.

An important advantage of this drying technique is a markedly reducedprocessing time. Whereas veneers typically require about 5 minutes in aconventional air dryer, processing times of as little as 20 seconds arepossible using the techniques of the present invention. This hasindirect benefits additional to the general desirability of reducingprocessing time. If high volume output is to be achieved in a reasonablysized conventional dryer, complex veneer input,

output and conveyor. mechanisms are needed to provide for thesimultaneous handling of many veneers within the dryer. These mechanismscan be-greatly simplified by utilizing the techniques of the presentinvention, in that high capacity output is possible in a.compact systemin which veneers may be continuously passed through the processingregion along a single path if desired.

Still another very important advantage of the invention is the moistureleveling effect which is inherent in the above described combination ofsteps. As the heating effect of microwave energy is primarily due tointeraction with the internal moisture of the veneers, heating anddrying is more intense at the relatively moist regions. Consequently,the finished veneers then have a much more uniform moisture content thanhas heretofore been the case.

Considering now suitable structurefor accomplishing the above describedoperations, the microwave irradiation regions 23 are established anddefined by a series of spaced apart parallel waveguide sections 26 whichtransect the path 22 of veneer travel and which in this instance are ofrectangular cross section. Toprovide for passage of the veneers throughthe waveguide sections 26, longitudinal slots, including input andoutput slots 27 and 28 respectively, are provided in the walls thereof.Provided that the waveguide sections 26 are excited in one of certainspecific modes, such as the TB 010 mode, slots 27 and 28 oriented inthis manner will not interfere with the operation of the waveguide.

The airflows at zones 24 are most advantageously defined by pairs of airknives 29 of which one pair is situated between each successive pair ofwaveguide sections 26 in parallel relationship thereto. The membersofeach pair of air knives 29 are disposed on opposite sides of the path oftravel 22 of the veneers and are tubular conduits 31 each having alongitudinal slit 32 facing the veneers to direct a planar jet of airthereagainst at high velocity.

in order to support the veneers and to carry the veneers along the pathof travel 22, roller conveyors 33 are utilized, one pair of rollersbeing disposed between each pair of air knives 29 and the subsequentwaveguide section 26 in parallel relationship therewith with the membersof each pair of rollers being on opposite sides of the veneers.

Thus the veneers are processed in accordance with the inventionby beingpassed through a repeated sequence of elements of which each unit iscomprised of a pair of conveyor rollers 33, a waveguide section.26, anda pair of air knives 29. The advantages of the invention are most fullyrealized if these elements are interrelated in a specific manner andhave certain structural characteristics which jointly contribute to thedesired results. A first very desirable characteristic is that thewaveguides 26 be formed of two separable members 34 and 36 with theboundary therebetween preferably lying in the plane defined by the pathof the veneers. The division between the two members 34 and 36 of eachwaveguide section 26 is thus similar to the input and output slots 27and 28 in that the transmission of energy through the waveguides is notinterfered with provided the-waveguides are excited in a compatiblemode. This split waveguide structure provides for easy access to theinterior of the waveguide sections 26 for the purpose of removing knots,sawdust, and other debris which may tend to accumulate therein and forother purposes.

Further advantages are realized by employingspecialized configurationsfor the input and output slots 27 and 28 through which the veneers passinto and out of the waveguide sections 26. If the width of these slotsin a direction normal to the veneers is a sizable portion of onewavelength of the microwave energy, and in the absence of correctivestructure to be hereinafter described, a substantial leakage of energythrough the slots can occur. To avoid this undesirable result, the inputslots 27 may be formed to have a width which isvery small in relation tothe microwave wavelength. However, with many veneers as well as othermaterials which may be treated in this apparatus, there may be -atendency to buckle and in the absence of corrective measures, scrapingor breakage of the veneers against the edges of the input slot 27 mightoccur where the slot is of very narrow width. To forestall this problem,the pair of roller conveyors 33 which precedes each waveguide section 26is situated very close to the input slot.

Electrical arcing-often causes substantial problems where divisions oropenings are present in the wall of a waveguide .and to avoid thisdifi'rculty, at the ends of the product input slots 27,.a tapered designis utilized in which the edges which define the slot converge graduallyas illustrated at 37.

The product output slots 28 of the waveguide sections 26 preferably have,a difi'ering construction for several reasons. First, the output slots28 cannot be made as narrow as the input slots 27 without difficultiesfrom jamming of the veneers inasmuch as it is not generally practical todispose a pair of the rollers 33 adjacent the output slot. Second, aswill hereinafter be discussed in more detail, it is desirable to disposethe subsequent pair of air knives 29 immediately adjacent the outputslot 28 and to electrically and mechanically connect the air knifestructure with the waveguide structure at the output slot. Further, itis desirable to provide for the directing of a portion of the gas flowfrom the air knives 29 into the waveguidesections 26 through the outputslots.28.

Accordingly the product output slots 28 are made relatively wide in adirection normal to the veneers and a lip 38 extends outward-from eachedge of the slot. The lips 38 are slightly convergent inthe exampleillustrated in FIG. 1 and in combination with the relatively wide widthof slot 28 facilitate the passage of the veneers out of the waveguidewithout jamming or damage. The lips 38 are comprised of conductivematerial and are preferably formed as an integral portion of thewaveguide section members 34 and 36. The lips 38 also form a shelfagainst which the adjacent air knife conduits 31 may be secured to forman integral assembly of each waveguide section member 34 and 36 and oneof the associated pair of air knives. This arrangement provides forlocating the slits 32 of the air knives 29 adjacent the outer edges ofthe lips 38 so that a substantial portion of the airflow will betransmitted into the adjacent waveguide section 26 through the spacebetween lips 38 and through output slot 28. This extends the zone ofinteraction between air flow and the veneers into the waveguide sections26 and provides for the scavenging of moisture from the interior of thewaveguide sections to facilitate drying and to reduce the possibility ofarcing therein. The exhaust of such air from the waveguide sections 26is provided for by a plurality of ap .rtures 39 in the walls thereofwhich are of sufficiently small diameter that no microwave leakage canoccur.

A very important function of the lips 38 is to suppress energy leakagethrough the relatively broad output slot 28. The field distributions ina waveguide of this type are such that currents travel in an axialdirection along the center of the broadwalls of the waveguide and thetransverse currents are zero at this point. Thus a longitudinal slot 28along the center of the broadwall does not interrupt currents andtherefore will not radiate energy provided the slot is very narrow. in awaveguide operating at 915 megacycles, for example, a slot having awidth less than approximately 1 inch will exhibit very little leakage.However, as the slot is made wider as may be desirable in the presentinstance for the reasons discussed above, the field distribution issignificantly modified and leakage may occur. Further, the presence of adielectric material, such as the veneers 21, within the slot, will tendto couple energy out of the slot. This effect is counteracted in thepresent invention by the lips 38. Energy which leaks through the slot 28couples into the space between the lips 38 in a higher order mode as ina parallel plane transmission line. If the lips 38 extend away from thewaveguide wall for approximately one-quarter wavelength or an oddmultiple thereof, maximum suppression of energy release is accomplished.Under this condition, the lips 38 act as quarter wave choke assemblieswith respect to energy propagation normal to the waveguide. In addition,any dielectric material between the lips 38, such as the veneers 21,aids in producing attenuation by absorbing energy in the space betweenthe lips.

In a 915 ,megacycle system for veneer drying, the width of the outputslots 28 may typically be 3 inches and the lips 38 may extend outwardfor a similar distance which is approximately one-fourth of the 13-inchfree space wavelength.

For maximum effectiveness, specialized constructions may be utilized ateach end of each pair of lips 38 to suppress leakage in these areas andto inhibit electrical arcing. The ends of the product output slot 28 inthe waveguide wall are tapered to converge gradually as in the case ofthe previously described input slots 27 and the lips 38 are arranged tobe similarly convergent but without being joined or closely spacedinasmuch as any such contact or near contact may promote sparking. Thismay be accomplished in several different ways, the lips 38 illustratedin FIG. 1 being convergent towards each other and being convergenttowards the midplane of the associated waveguide section 26 asillustrated at 41. FIG. 2 illustrates a modified construction for thelips. In this modified embodiment the waveguide section 26a is againformed of two separable members 34 and 36' which are divisible along acentral longitudinal plane through the waveguide. The lips 38' in thisexample are parallel rather than convergent and extend outward from thewaveguidesection 26a for a uniform distance at all points along each lipincluding the end sections 41 thereof. The lips 38' at end sections 41'converge in correspondence with the convergent tapered end of outputslot 28 but are terminated short of the apex thereof so that the ends ofthe lips are spaced apart forming a gap sufficiently wide to avoidarcing. In a waveguide construction having the dimensions discussedabove, wherein the central portions of the lips 38' are separated by a3-inch gap, the spacings of the ends of the lips 38' may typically beabout 1 inch.

Referring now again to FIG. 1, by forming the air knife conduits 31 ofelectrically conducting material and by securing the air knives to theadjacent waveguide sections 26 by electrically conductive means such asby welding, further suppression of sparking is accomplished and the airknives are in effect an extension of the lips 38 to assist in theseveral functions thereof. In this connection, since the length of thelips 38 should be one-quarter wavelength to suppress microwave emissionas described above, the air knives should be located so that the slits32 are situated one-quarter wavelength away from the wall of theassociated waveguide. This is desirable since it is the air knife slit32 which defines the end of the associated lip 38 insofar as electricaleffects are concerned.

Considering now the detailed construction of a veneer dryer inaccordance with the invention, with reference to F108. 3 and 4 incombination, there is shown a generally rectangular cabinet 43 having aproduct input end 44 and product output end 46. As best shown in FIG. 4in particular, cabinet 43 is formed of electrically conductive walls 47to aid in confining stray microwave energy. To reduce air heating cost,the cabinet walls 47 are formed of a double layer of electricalconductor with a suitable thermal insulation 48 disposed therebetweenexcept at the product input and output sections 44 and 46. The walls 47ofthe input and output sections may be a single layer of electricalconductor as transverse internal walls 49, also formed of thermalinsulation disposed between electrically conducting layers, are situatedbetween the product input and output sections and the central section 52of the cabinet.

The walls of cabinet 43 may be attached to and supported by suitablestructural members 51 and the sidewalls of the central section 52 of thecabinet are preferably formed as a continuous series of doors 53supported by hinges 54 and having latches 56 to provide for convenientaccess to the interior of the cabinet 43 at any point therealong oneither side thereof. To insure good electrical contact between the edgesof each door 53 and the adjacent fixed portion of walls 47 and toprevent air leakage therebetween, R.F. seals, such as copper gaskets 57,are disposed thereat.

The product input and output end sections 44 and 46 respectively ofcabinet 43 provide for access for feeding veneers into the cabinet andfor passing the veneers out of the cabinet while suppressing theemission of stray microwave energy through the necessary openings. Toattenuate the microwave energy both the input section 44 and the outputsection 46 are loaded by means of a pair of flattened helicalnonconductive coils 58 containing water or other electrically lossymaterial, the coils being supported by transverse tubular members 59formed of dielectric material.

Considering now the detailed construction of the product input section44 of cabinet 43 and with reference to FIGS. 4 and 5 in combination, aslot opening 61 in the conducting end wall 62 of cabinet 43 provides foradmittance of the veneers and is of sufficient height to avoid anydifficulties from buckling of the veneers. To facilitate the feeding ofveneers through slot 61, a short shelf 63 extends outwardly therefrom. Agenerally similar slot opening 64 provides for passage of the veneersthrough the internal thermally insulated end wall 49. To facilitate thepassage of the veneers through both slots 61 and 64 and to inhibit thepassage of microwave therethrough, a conducting sloping lip 66 may beprovided along the upper edge of each slot with the lips extending in adirection corresponding to the travel of the veneers and being convergedtowards the plane defined by the veneer path. To support the veneers inpassage between slots 61 and 64, a flat dielectric member 67 extendstherebetween beneath the veneer path. The pair of helical water-loadedcoils 58 are vertically spaced within product input section 44 with onebeing above the veneer path and the other being below the path and belowmember 67. The effect of the water or other lossy substance containedwithin coils 58 is to absorb and attenuate microwave energy attemptingto pass from slot 64 to slot 61. Only a very small proportion of suchenergy will be directed parallel to the path of the veneers in a waywhich will enable the energy to pass from slot 64 to slot 61 withouttraversing the water within coils 58. Most such energy will be directedobliquely with respect to the veneer path and will be repeatedlyreflected between the conducting walls of the product input section 44before reaching the outermost slot 61 and accordingly will make multiplepasses through the lossy coils 58 and will be attenuated thereby. Asystem of this general type for suppressing microwave emission throughan opening in a multimode microwave chamber is described in detail incopending application Ser. No. 589,149 of Morris R. .Ieppson forApparatus and Process for Microwave Treatment filed Sept. 7, 1966 andassigned to the assignee of the present application.

Referring now to FIG. 9, the product output end section 46 of cabinet 43also has electrically conducting walls 47 and slot openings 69 and 71 inthe end wall and internal thermally insulated wall 49 respectively. Toguide the veneers into the output end section 46, upper and lowerelectrically conductive members 72 are disposed in slot opening 71 andare convergent in the direction of veneer travel. For similar purposes,flat dielectric members 73 extend from the upper and lower ones of themembers 72 to the upper and lower edges of slot 69 at the end wall ofthe product output end section. The pair of water-loaded coils 58 of theproduct output end section are disposed one below and one above thedielectric members 73 to attenuate stray microwave energy ashereinbefore described.

Referring now again to FIG. 4, the waveguide sections 26 and air knives29 which provide the drying effect are situated within the centralsection 52 of cabinet 43 between the internal thermally insulated walls49. Two meander waveguide assemblies, including a first waveguideassembly 76 and second waveguide assembly 77, are disposed at the pathof travel of the veneers within cabinet 43, sections of the firstwaveguide assembly being designated in FIGS. 4 to 9 of the drawings byreference numeral 26 and sections of second waveguide assembly beingdesignated by number 26". All sections 26 and 26" of both waveguideassemblies are parallel and spaced apart along the veneer path intransecting relationship thereto.

Advantageous results are realized by alternating pairs of the sections26 of the first waveguide assembly 76 with pairs of sections '26" of thesecond waveguide assembly. This arrangement provides for a mechanicallysimple overall construction in that the successive sections of each ofthe waveguide assemblies 76 and 77 may be coupled at the sides of theveneer path by U-shaped side connectors 78 and 78' which may be coplanarwith the primary waveguide sections 26' and 26". In some other possiblesequences of waveguide sections in a system having a plurality ofmeander waveguide assemblies, much more complicated coupling structureis needed at the sides of the assemblies. In some such arrangements, forexample, such connectors would have to extend vertically in order to goover or under adjacent sections of the other waveguide assembly. Stillanother advantage of this sequencing of waveguide sections 26' and 26"is that the diminishing microwave intensity along each waveguideassembly 76 and 77 from the power input end towards the opposite endthereof is correlated with the diminishing moisture content of theveneers as the veneers travel through the dryer. Thus, the veneers areexposed to a progressively diminishing microwave power level in passagefrom the input to the output end of the dryer. In other possiblearrangements utilizing multiple meander waveguide assemblies ofthis"general type, regions of relatively strong microwave field mustnecessarily follow regions of lesser power density.

Referring now to FIG. 6 in combination with FIG. 4, microwave power issupplied to the two waveguide assemblies 76 and 77 through inputwaveguide sections 79 and 81 respectively which extend upwardly, throughthe top wall 80 of cabinet 43, from the forward end of the waveguideassemblies at opposite sides of the veneer path to connect with twoseparate microwave sources-82 and 83 respectively as shown in FIG.-.3.By providing for power input to the two waveguide assemblies 76 and 77at opposite sides of the veneer path and at the initial waveguidesections 26' and 26" thereof, the energy density gradient at the veneersalong sections 26' of one assembly is compensated for by the reversedbut equal magnitude gradients along the sections 26" of the other assemland a more uniform drying effect is accomplished across t veneers.

Near the output end section 46 of the cabinet, waveguide end sections 84and 86 extend upwardly through the top wall of the cabinet to connectwaveguide assemblies 76 and 77 with conventional water loaded waveguideterminations 87 and 88 respectively, also shown in FIG. 3.

Referring now to FIGS. 5 and 6, each of the waveguide sections 26' and26" has the slot and lip construction hereinbefore described to providefor the passage of the veneers therethrough. This includes therelatively narrow slot openings 27 with a tapered end configuration 37for admitting the veneers and the wider slot openings 28 through whichveneers emerge from the waveguide sections, the slot openings 28 havingthe previously described lips 38.

Each waveguide section 26' and 26" is formed of an upper and lowermember 34 and 36 respectively which are separable at the plane definedby the veneer path as hereinbefore described to provide for access tothe interior of the waveguide structure and to expose the veneers intransit therethrough if desired. For such purposes, it is highlyadvantageous if the upper members 34 of all waveguide sections can belifted away from the lower members 36 simultaneously and by motorizedmeans. Referring still to FIGS. 5 and 6 in conjunction, this is providedfor by securing the upper member 34 of all waveguide sections to asupradjacent rectangular frame 85. Frame carrying all of the upperwaveguide sections members 34 is liftable directly upward to a positionadjacent the underside of the top wall 80 of the dryer as indicated bydash lines at 85 in FIG. 5. To provide for this movement, the U-shapedside connectors 78 and 78' which couple successive waveguide sections26' and 26" are also divisible along the plane defined by the veneerpath so that the upper half of each such connector moves with frame 85.

To guide the frame 85 during this movement, four vertical guide posts 89are mounted within the cabinet 43 in proximity to the four corners ofthe frame and extend through guide passages 91 therein. To effect theupward and downward movement of the frame 85, a vertical screw 92 issituated adjacent each of the posts 89 and such screw engages a threadedbushing 93 in frame 85 to lift the frame along posts 89 as the screwsare turned. It will be appreciated that the rotation of each of the fourscrews 92 must be carefully synchronized;to avoid misalignment andpossible jamming of the frame 85. For this purpose, a gear 94 is mountedat the top of each screw 92 above cabinet top wall 80 and all four gears94 are coupled by a chain 96 which extends through a drive motor-housing97 on the top of the cabinet 43. Referring now to FIG. 8, housing 97 hasslots 98 at opposite ends to provide for passage of the chain 96therethrough and a loop is formed in the chain therein by a pair ofidlers 99 and drive gear 101. Gear 101 is turned by an electrical drivemotor 102 through a speed reduction device 103. Thus, the frame 85 andupper members 34 of the waveguide sections may be lifted and lowered asdesired by operation of the motor 102.

Referring now to FIG. 5 in particular, the lower members 36 of thewaveguide sections are stationary and are secured to a pair of beams 104which extend longitudinally through the central section 52 of the dryerimmediately below the waveguide sections. To facilitate the removal ofcondensed moisture, a drain pan 106 is disposed between the beams 104below the waveguide assemblies and, as shown in FIG. 6, is provided withan outlet 107 to a drain trough 108.

Inasmuch as the lower waveguide section members 36 are fixed inposition, the coupling to the microwave power input waveguide sections81 and 86 may be made through elbowshaped 'Lwaveguide portions 109 whichhave conventional flange couplings 111, including bolts 113, to thelower members 36 of the initial sections 26' and 26" of the twowaveguide assemblies. However, as hereinbefore described, the uppermembers 34 of the initial waveguide sections may be lifted in an upwarddirection as illustrated in FIG. or lowered against the lower waveguidesection members 36 as illustrated in FIG. 11. Thus, the type of flangedjoint used to couple lower members 36 to elbows 109 is not adaptable tocoupling the upper members 34 to the elbows. Such a coupling wouldrequire the disengagement of bolts before the upper waveguide member 34could be retracted and the reengagement of such bolts when the upperwaveguide section inembt was lowered. Further, the requirement that suchflanges be abutted for good electrical contact would require an extremedegree of precision in the mechanism which raises, lowers and guides theupper waveguide section members 34. To avoid these problems, the uppermember 34 of the initial section of each of the two waveguide assemblies76 and 77 is not mechanically coupled to the corresponding portion ofthe associated elbow 109 and a small clearance space 114 is presenttherebetween as best shown in FIGS. 11 and 12. Notwithstanding the lackof physical contact, electrical continuity between the elbow 109 andupper waveguide section member 34 is established by a specialized chokecoupling I12.

Considering now the construction of the choke coupling 112, withreference to FIGS. 10 to 12 in conjunction, the upper waveguide sectionmember 34 has a flange 116 which includes a relatively thick baseportion 117 adjacent the waveguide section member and further includesthin rectangular side flange projections 118 which are integraltherewith and which extend normal to the waveguide axis to define oneboundary of the hereinbefore described clearance space 114. To definethe opposite boundary of the clearance space 114, a pair of flangeprojections 119 extend sidewardly from the adjacent end of elbow 109 inparallel relationship to flange projections 118. As best shown in FIG.12 in particular, both the flange projections 118 of waveguide sectionmember 34 and flange projections 119 of elbow 109 extend outwardly for adistance slightly greater than one-quarter wavelength of the microwaveenergy which is transmitted through the system.

Each side flange projection 119 of the elbow 109 is formed with a gap121, as best illustrated in FIG. 12, with the center of the gap beingsituated substantially one-quarter wavelength outward from the innersurface of the adjacent wall of elbow I09. The gaps 121 are defined byconvolutions 122 in the flange projections 119 which extend parallel tothe adjacent wall of the elbow 109 and the gaps have a length, measuredfrom the center of the clearance space 114, of substantially one-quarterwavelength. It should be noted that short outer sections 123 of theflange projections 119 extend outward from the gap 121 for a smalldistance to conform with the facing flange projections 118 of member 34.

The top and bottom of gap 121 are closed and the flanged structure atelbow 109 strengthened, by wing plates 124 which extend outwardly fromthe elbow above and below the flange projections I19 and which aresecured thereto by electrically conductive means such as by soldering.Flange projections 118 and 119 as well as wing plates 124 are all formedof electrical conductor. Thus both clearance spaces 114 and gaps 121 aredefined and bounded by electrically conductive means.

Referring now to FIG. 10 in particular, difficulties from slightmisalignments during lowering of the movable waveguide section member 34towards lower waveguide section member 36 are avoided by a rampprojection 126 which extends upward from the end of elbow 109. Rampprojection 126 has a tapered surface 127 facing the movable waveguidesection member 34 and carries a roller wheel 128 at each side to engagethe flange base portion 117 of movable member 34 as the movable memberapproaches the fixed waveguide section member 36. Rollers 128 aresituated above the level of movable member flange 116 when the movablemember is at the fully lowered position thereof so that no physicalcontact exists at such time.

Microwave energy is transmitted from elbow 109 to the fixed lowerwaveguide section member 36 in the usual manner in that flanges ll]define a direct electrically conductive connection therebetween andinsofar as wall currents and field configurations are concerned the twomembers are in effect an integral conductor. Considering now the mannerin which the microwave energy is transmitted from the elbow 109 to theupper waveguide section member 34 despite the clearance space 114therebetween, the configuration of the flange projections 118 and 119,including gaps 121, causes the flange structure to behave electricallyin a manner equivalent to a direct conducting connection between elbow109 and member 34. Microwave energy being transmitted along the elbow109 is coupled into the inner portion of each clearance space 114; andsince the clearance space extends outward for a distance greater thanone-quarter wavelength, field nodes occur at the entrances to gaps 121.Inasmuch as the gaps I2I have a depth of substantially one-quarterwavelength and are terminated by electrical conductors 129, energy iscoupled into the gaps rather than being transmitted further alongclearance spaces 114 where a much greater impedance is present. Thus,the conductive connections 129 across the ends of gaps 121 constituteshort circuits which are situated onehalf wavelength from the waveguidewalls and therefore reflect energy back towards the waveguide. This, ineffect, creates a virtual short across the inner end of each clearancespace 114 at the inner walls of elbow 109 and waveguide section member34.

Referring now again to FIG. 4, it may be seen that an additional similarspecialized choke coupling 112 is employed at the output end of eachwaveguide assembly 76 and 77 to couple the final section 26 of eachassembly to the fixed sections 84 and 86 which extend upwardly to thewaveguide terminations 87 and 88 as previously described with referenceto FIG. 3.

Considering now the detailed construction of the veneer conveyormechanism, with reference to FIGS. 6 and 9 in conjunction, an upperconveyor roller 33' and a lower roller 33 are disposed adjacent theveneer input slot 27 of each waveguide section 26 in parallelrelationship thereto to support and drive the veneers as hereinbeforedescribed. Both rollers 33 and 33" of each such pair are mounted onbrackets 131 which extend upward from beams 104 at the ends of each pairof rollers. The lower roller 33" of each pair is journaled in theassociated bracket 131 for rotation about a fixed axis while the upperrollers 33' are engaged in vertical slots 132 at the top of the bracketswhereby the upper rollers may lift or drop as necessary in response tovariations in the thickness of veneers. Each lower roller 33" has adrive gear 133 at one end and an additional gear 134 at the opposite endwhich engages a gear 136 at the corresponding end of the associatedupper roller 33 to transmit rotary motion thereto.

As shown in FIG. 9 in particular, all lower rollers 33" are driven by asingle continuous chain 137 which is operated by a motor 138 situated inthe product output end section 46 of the dryer. Chain 137 extends frommotor 138 along beam 104 to a tensioner assembly 139 in the productinput end section 44 of the dryer, slots 141 being provided in theinternal thermally insulated walls 49 to provide for passage of thechain therethrough. At tensioner assembly 139 the chain engages twospaced idlers 142 which have fixed axes of rotation and the portion ofthe chain which extends between the two idlers passes around a thirdidler 143 which is movable. An adjustable tensioner spring 144 draws themovable idler I43 downwardly from idlers 142 to maintain the chain undera selected degree of tension.

At each of the brackets 13] along beam 104, chain 137 engages two spacedidlers 146 situated above the chain and a gear 146 which is situatedtherebetween but below the chain. Gear 146' turns a coaxial gear 145which is coupled to roller drive gear 133 by a continuous chain 150.

Considering now the detailed structure of the air circulation system,with reference again to FIG. 9, a pair of the air knives 29 are disposedadjacent the veneer output slot 28 of each waveguide section 26 in themanner hereinbefore described. In particular, the upper air knifeconduit 31 and lower air knife conduit 31' are secured to the upper andlower members 34 and 36 respectively of the associated waveguidesections 26 at the lips 38 of the output slot 28 and each has a slitopening 32 to direct air against the adjacent surface of the veneers. Asshown in FIG. 3, air is pumped to the air knives by three air blowers147 which are disposed along one side of the dryer near the base thereofwith each being operated by a separate electrical motor 148.

Referring now to FIG. 6, an angled wall 149 of thermally insulatedmaterial separates the blowers 147 and motors 148 from the interiorregion of the dryer and each blower has an air intake conduit 151 whichextends upwardly through such wall. Thus each such blower 147 draws airwhich has been exhausted from the air knives for recirculation thereto,thereby minimizing air heating costs. As shown in FIG. 3, an air heater152 which may be of the type having a gas flame 153 therein is disposedbetween each air intake 151 and the associated blower 147. Since theoxygen supply in the recirculating air may be diminished, each intakeconduit 151 also has air inlet openings 155 for admitting a selectedamount of external air to heaters 152 to support combustion at flames153.

Referring now again to FIG. 6 in particular, the heated air outlet 154of each blower 147 extends through wall 149 to communicate with amanifold conduit 156 which extends longitudinally within the dryer belowdrain pan 108. A transverse rectangular air conduit 157 below each pairof air knives 29 transmits heated air thereto from manifold 156. Tocouple each such transverse conduit 157 to the associated pair of airknives, an elbow 158 is disposed at the end opposite manifold 156.Manifold conduit 156 may supply all air knives 29 from the combinedoutputs of the three blowers 147. However, it is frequently advantageousto adjust air temperature or velocity or both to different values atdifferent stages along the veneer path. For this purpose, partitions 160may be disposed in the manifold conduit to confine the output of eachblower to a separate group of air knives. Referring now to FIG. 7, thelowermost air knife conduit 31' of each pair has an angled end portion159 which extends down to connect with the associated elbow 158.However, the retractability of the upper air knife conduit 31 requires acoupling to the elbow which automatically engages and disengages ifcomplications are to be avoided in connection with the retracting of theupper waveguide member 34 as hereinbefore described. For this purpose,the end portion 161 of the upper air knife conduit 31 is angleddownwardly and provided with an enlarged lower end 162, having areentrant lip 163. A sleeve 164 projects upward from elbow 158 and isreceived in end 162 of the upper knife conduit 31 when the air knife islowered to the operating position thereof.

To avoid a progressive accumulation of moisture in the recirculatingairflow, a selected proportion of the output of blowers 147 is exhaustedfrom the dryer. Referring to FIGS. 6 and 9, exhaust conduits 166 extendbeneath beams 104 from manifold 156 to a vertical exhaust stack 167 atthe opposite side of dryer. To control the proportion of the output ofthe blowers 147 which is exhausted rather than being recirculated to theair knives, a valve 168 of the damper type is situated within stack 167.

In operation, with reference to FIG. 5, veneers are continuously fedonto shelf 63 at the product input end section 44 of the dryer and arepassed through slots 61 and 64 thereof to the initial pair of conveyorrollers 33. The successive pairs of rollers 33 then carry the veneersthrough successive ones of the waveguide sections 26 and betweensuccessive pairs of air knives 29. At each passage through a waveguidesection 26 internal heat is rapidly generated within the veneers drivingthe moisture content outward toward the veneer surfaces. Moisturewhichaccumulates at the veneer surfaces at each waveguide section 26 isthen removed by the subsequent pair of air knives 29 with the airflowtherefrom having the further beneficial effect of supplying additionalheat to the veneers. Thus the veneers become progressively dryer whilepassing through the cabinet 43 and ultimately are carried out of thedryer between the members 73 of the product output end section 46. Ashas been previously discussed, the drying effect is proportionatelygreater in the more moist areas of the veneers so that the finishedproduct has a uniform low moisture content.

Control of the degree of drying to accommodate to variations betweenbatches of green veneers and other variable factors may be effected byadjusting one or more of several operating parameters. These include themicrowave power input to one or both of the meander waveguide assemblies76 and 77, airflow velocity and temperature as determined by blowers 147and heater 152 respectively in conjunction with valve 168 and conveyorspeed as determined by motor 138. Process control is facilitated by avery significant characteristic of the above described system.Specifically, a curve of the moisture content of the veneers plottedagainst time (or longitudinal position within the dryer) ischaracterized by periods of rapidly diminishing moisture alternated withplateaus at which moisture tends to remain fairly constant. Thus alocalized area of a veneer which is at one such plateau may temporarilyundergo little further drying while more moist areas are rapidly broughtdown to the plateau. For many woods, one such plateau occurs at amoisture content of about 6 percent to 7 percent and in many instancesthis is a very suitable moisture level for bonding the veneers to formplywood provided that the veneers have been processed in accordance withthe present invention.

In one example of the practice of the above described method, partiallydried veneers comprising is, 54-inch by 10 I inch pieces of 0.1-inchthick douglas fir were conveyed through a dryer of the type describedabove in which a sequence of 19 spaced-apart waveguide sections wereenergized from two separate 28 kilowatt microwave sources, operating ata frequency of 915 megacycles. The veneers were subjected to air flowsbetween successive waveguide sections which had a temperature of 300 F.and a velocity at the air knife slits of 3,000 feet per minute. Veneertransit time through the system was 48 seconds. Prior to this treatment,the veneers had an average moisture content of 8.1 percent withsubstantial variations at different areas of individual ones of theveneers and sizable differences in the moisture content of separateveneers the standard deviation recorded being 4.9. After drying asdescribed above, all veneers had an average moisture content of 3.9percent and localized variations in the moisture content of individualveneers did not exceed about plus or minus 1 percent for which astandard deviation of 0.4 was recorded. No significant surfacedeterioration from the process was discernible.

While the invention has been described with respect to specificembodiments, it will be apparent that numerous variations andmodifications are possible and it is not intended to limit the inventionexcept as defined in the following claims.

We claim:

1. In a meander waveguide-apparatus for drying materials which are movedalong a plane path, the combination comprising first and second meanderwaveguide assemblies each having a plurality of spaced-apart sectionsextending across said plane path, pairs of successive sections of saidfirst waveguide assembly being alternated with pairs of successivesections of said second waveguide assembly along said path.

2. The combination defined in claim 1 further comprising first andsecond groups of U-shaped waveguide connectors, the connectors of saidsecond group being of greater length and radius than the connectors ofsaid first group and wherein'the sections of said first waveguideassembly are coupled by connectors of said first group at a first sideof said plane path and are coupled by connectors of said second group atthe opposite second side of said plane path, and

wherein said sections of said second waveguide assembly are coupled byconnectors of said second group at said first side of said path and arecoupled by connectors of said first group at said second other sidethereof.

3 The combination defined in claim 1 further comprising a pair ofmicrowave energy sources, and wherein each of said waveguide assembliesis energized by a separate one of said sources and at opposite sides ofsaid path.

4. The combination defined in claim 1 comprising at least seven of saidwaveguide sections and wherein said first meander waveguide is comprisedof the first and third and fourth and seventh of said waveguide sectionsand wherein the second of said meander waveguide assemblies is comprisedof the second and fifth and sixth of said waveguide sections.

5. Apparatus for heating sheet materials wherein said materials areconveyed along a plane path and having a meander waveguide assembly withsections which extend across said plane path at spaced-apart locationstherealong, said waveguide assembly being formed of at least twoseparable components having the juncture therebetween at said planepath, a plurality of linear guide means extending in a directiongenerally normal to said plane path, one of said separable components ofsaid waveguide assembly engaging said guide means and being slidabletherealong, said apparatus further comprising power means for retractingone of said separable of said waveguide assembly away from the otherthereof.

6. The apparatus defined in claim 5 wherein said power means forretracting said one of said separable components of said waveguideassembly comprises at least one rotatable lead screw extending parallelto said guide means and engaging said one of said separable componentsof said waveguide assembly and a motor for electrically rotating saidlead screw.

7. An apparatus forheating sheet materials wherein said materials areconveyed along a plane path and having a meander waveguide assembly withsections which extend across said plane path at spaced-apart locationstherealong, said waveguide assembly being formed of first and secondseparable members having the juncture therebetween at said plane path,one end of said waveguide coupled to a microwave source through a fixedwaveguide portion, said first waveguide member having an end spaced asmall distance from the end of said fixed waveguide portion and beingelectrically connected thereto through a choke coupling comprising afirst electrically conducting flange secured to said end of said firstwaveguide member, a second electrically conducting flange secured tosaid end of said fixed waveguide portion and being spaced from saidfirst flange to define a clearance space having an effective electricallength of one-half wavelength and said second waveguide member and saidfixed waveguide portions being secured together to be electrically andmechanically continuous.

1. In a meander waveguide apparatus for drying materials which are movedalong a plane path, the combination comprising first and second meanderwaveguide assemblies each having a plurality of spaced-apart sectionsextending across said plane path, pairs of successive sections of saidfirst waveguide assembly being alternated with pairs of successivesections of said second waveguide assembly along said path.
 2. Thecombination defined in claim 1 further comprising first and secondgroups of U-shaped waveguide connectors, the connectors of said secondgroup being of greater length and radius than the connectors of saidfirst group and wherein the sections of said first waveguide assemblyare coupled by connectors of said first group at a first side of saidplane path and are coupled by connectors of said second group at theopposite second side of said plane path, and wherein said sections ofsaid second waveguide assembly are coupled by connectors of said secondgroup at said first side of said path and are coupled by connectors ofsaid first group at said second other side thereof.
 3. The combinationdefined in claim 1 further comprising a pair of microwave energysources, and wherein each of said waveguide assemblies is energized by aseparate one of said sources and at opposite sides of said path.
 4. Thecombination defined in claim 1 comprising at least seven of saidwaveguide sections and wherein said first meander waveguide is comprisedof the first and third and fourth and seventh of said waveguide sectionsand wherein the second of said meander waveguide assemblies is comprisedof the second and fifth and sixth of said waveguide sections. 5.Apparatus for heating sheet materials wherein said materials areconveyed along a plane path and having a meander waveguide assembly withsections which extend across said plane path at spaced-apart locationstherealong, said waveguide assembly being formed of at least twoseparable components having the juncture therebetween at said planepath, a plurality of linear guide means extending in a directiongenerally normal to said plane path, one of said separable components ofsaid waveguide assembly engaging said guide means and being slidabletherealong, said apparatus further comprising power means for retractingone of said separable of said waveguide assembly away from the otherthereof.
 6. The apparatus defined in claim 5 wherein said power meansfor retracting said one of said separable components of said waveguideassembly comprises at least one rotatable lead screw extending parallelto said guide means and engaging said one of said separable componentsof said waveguide assembly and a motor for electrically rotating saidlead screw.
 7. An apparatus for heating sheet materials wherein saidmaterials are conveyed along a plane path and having a meander waveguideassembly with sections which extend across said plane path atspaced-apart locations therealong, said waveguide assembly being formedof first and second separable members having the juncture therebetweenat said plane path, one end of said waveguide coupled to a microwavesource through a fixed waveguide portion, said first waveguide memberhaving an end spaced a small distance from the end of said fixedwaveguide portion and being electrically connected thereto through achoKe coupling comprising a first electrically conducting flange securedto said end of said first waveguide member, a second electricallyconducting flange secured to said end of said fixed waveguide portionand being spaced from said first flange to define a clearance spacehaving an effective electrical length of one half wavelength and saidsecond waveguide member and said fixed waveguide portions being securedtogether to be electrically and mechanically continuous.